In nonvolatile memory, such as NAND-type electrically erasable programmable read-only memory (EEPROM), along with the miniaturization thereof, the inter-cell interference effect (proximity interference effect) is increasing. The increase in inter-cell interference effect may allow the threshold voltage of a memory cell to be changed as a result of writing to the adjacent memory cell, so that the threshold voltage distribution of memory cells may be widened.
On the other hand, memory cells capable of storing multi-valued information, such as a multi-level cell (MLC), have been developed. In an MLC, since the threshold voltage range of each level is relatively narrow, it is desirable that the threshold voltage distribution of memory cells be as narrow as possible. However, widening of the threshold voltage distribution of memory cells due to the inter-cell interference effect may make it difficult to keep the threshold voltage distribution of memory cells within the threshold voltage ranges of the MLC. Furthermore, narrowing the threshold voltage distribution of memory cells in consideration of the inter-cell interference effect may cause the writing time to increase.